Internal exhaust recirculation method for an internal combustion engine

ABSTRACT

The invention relates to the internal exhaust-gas recirculation in an internal combustion engine with gas exchange valves periodically controlled by a camshaft ( 22 ) in accordance with the four-stroke principle, in which, during the gas exchange exhausting of exhaust gas from the respective cylinder ( 1 ) into the exhaust duct ( 5 ), an intake valve ( 2 ) associated with the respective cylinder ( 1 ) is open in a crank angle range after top dead center of the ignition of the gas mixture present in the cylinder ( 1 ), in order to exhaust part of the exhaust gas into an intake duct ( 3 ), so that during the next gas exchange intake both fresh gas and exhaust gas are taken in from the intake duct ( 3 ), the respective intake valve ( 2 ) being actuated during the gas exchange exhausting independently of the periodic gas exchange intake, and the additional opening of the respective intake valve ( 2 ) being carried out in the range from 110 to 150% of the opening angle of the start of opening of the exhaust duct ( 5 ) by the exhaust valve ( 4 ).

BACKGROUND OF THE INVENTION

The invention relates to a method for internal exhaust-gas recirculationand to a correspondingly designed internal combustion engine.

Throughout the world, the demands imposed on the purity of the exhaustgases from internal combustion engines are subject to increasingstatutory control, which is becoming ever more stringent. One possibleway of satisfying these demands with regard to NO_(x) emissions consistsin internal exhaust-gas recirculation. This involves a certain quantityof exhaust gas being admixed with the fresh gas in a cylinder of theinternal combustion engine prior to ignition. The NO_(x) emission can beconsiderably reduced if an optimum mixing ratio is used.

To produce the mixture, DE 34 01 362 C2 has disclosed a method forinternal exhaust-gas recirculation in an internal combustion engine withgas exchange valves periodically controlled by a camshaft in accordancewith the four-stroke principle, in which the quantity of exhaust gaswhich remains in addition to the fresh gas after the gas exchange in thecylinder has ended can be adjusted by means of the opening and closingtimes of the gas exchange valves. If, for example, an exhaust is notclosed after exhausting of exhaust gas from the cylinder has concluded,but rather is kept open for an adjustable period of time during thesubsequent intake stroke, a quantity of exhaust gas, which depends onthis period of time, can be taken in together with fresh gas.Consequently, the opening and closing times of the gas exchange valvessimultaneously control the gas exchange and the internal exhaust-gasrecirculation. Their implementation therefore constitutes a compromisebetween optimum gas exchange and optimum internal exhaust-gasrecirculation.

WO 03/040540 has disclosed a method for internal exhaust-gasrecirculation in which an intake valve is open during the gas exchangeexhausting of exhaust gas from the cylinder into the exhaust duct, inorder for part of the exhaust gas to be exhausted into the intake duct,so that both fresh gas and exhaust gas are taken in from the intake ductduring the next gas exchange intake. In this case, the intake valve isactuated during the gas exchange exhausting independently of theperiodic gas exchange intake, with the start of the primary stroke ofthe intake valve lying in the range between 180° and 210° and the end ofthis primary stroke lying in the range from approximately 270° to 310°crank angle after top dead center of the ignition. However, this onlyallows moderate exhaust-gas recirculation rates at full load.

SUMMARY OF THE INVENTION

Consequently, the invention is based on the object of providing internalexhaust-gas recirculation in which very high exhaust-gas recirculationrates are possible at full load and low or even zero exhaust-gasrecirculation rates are possible at low engine speeds.

Consequently, the invention provides internal exhaust-gas recirculationin an internal combustion engine with gas exchange valves periodicallycontrolled by a camshaft in accordance with the four-stroke principle,in which, during the gas exchange exhausting of exhaust gas from therespective cylinder into the exhaust duct, an intake valve associatedwith the respective cylinder is open in a crank angle range after topdead center of the ignition of the gas mixture present in the cylinder,in order to exhaust part of the exhaust gas into an intake duct, so thatduring the next gas exchange intake both fresh gas and exhaust gas aretaken in from the intake duct, the respective intake valve beingactuated during the gas exchange exhausting independently of theperiodic gas exchange intake, and the additional opening of therespective intake valve being carried out in the range from 110 to 150%of the opening angle of the start of opening of the exhaust duct by theexhaust valve.

Contrary to expectations of the person skilled in the art, it hasemerged that it is possible and at the same time expedient if exhaustgas can be passed into the intake duct in the above mentioned range,which is well before 180° crank angle (CA) after top dead center of theignition (180° CA is the bottom dead center following top dead center ofthe ignition), i.e. at most at approximately 160° CA, and preferably atapproximately 110 to 150° CA after top dead center of the ignition, andif this exhaust gas can be kept in the intake duct even at high enginespeeds. On the other hand, at low engine speeds, on account of theconsiderably greater relief through expansion, pressure conditionsresult in which the exhaust gas which has been transported into theintake duct immediately flows back again, is exhausted into the exhaustduct during the subsequent exhausting phase and therefore does notparticipate in additional exhaust-gas recirculation from the intake ductduring the intake phase. On account of the fact that the intake valveopening and exhaust valve opening can be set independently in the aboveway, it is possible to exploit the quantities of exhaust gas present inthe cylinder to the extent required to lower the NO_(x) rates todesired, statutory values. As a result, it is also possible to effectrelatively simple retrofitting in order to achieve these values at alater stage. According to the invention, no exhaust gas is sucked backout of the exhaust pipe, since this exhaust gas does not have sufficientpotential to lower the NO_(x) rates and moreover would excessivelyaffect a turbocharger. Irrespective of this, relatively high exhaust-gasrecirculation in the lower engine speed range leads to increased smokenumbers on account of the reduced air/fuel ratio which results, whereasaccording to the invention these increased smoke numbers can be avoided.

The method according to the invention has a beneficial effect inparticular in commercial vehicle engines in the full-load range withvery high exhaust-gas recirculation rates.

The invention uses fixed cams of the camshaft and one primary oradditional cam, with the pressure conditions being such that at lowengine speeds after exhaust valve opening, the pressure has alreadydropped, so that soon after the intake valve is opened by means of theprimary cam the direction of flow is reversed, consequently the exhaustgas in the intake duct is pushed back and also flushed out before themain cam opens the intake valve and closes the exhaust valve. Bycontrast, if the main cam alone is used, as is customary in the priorart, on account of its contour applying equally to all load points, thequantity of recirculated exhaust gas is dependent on load point andtherefore also dependent on engine speed, but is fixed, especially sincethe parameters maximum valve stroke, opening duration and relativepositioning define the quantity of exhaust gas transferred into theintake duct.

In addition, during the intake of fresh gas from an intake duct into acylinder, an exhaust valve can be opened, in order in addition to takein exhaust gas from an exhaust duct, with the exhaust valve also beingactuated during intake independently of the periodic exhausting.

Decoupling of the gas exchange from the internal exhaust-gasrecirculation allows both operations to be optimally and independentlymatched to the particular requirements. Limits on the extent to whichthe exhaust-gas recirculation can be optimized by means of the openingand closing times and the stroke of the respective valve arise only fromthe pressure conditions present in the cylinder, which under certaincircumstances do not allow valves to be opened or closed at any time,and possibly from the design of the internal combustion engine and inparticular of the valve gear components.

As a result, the exhaust-gas recirculation rates which can be achievedat full load are higher than in the prior art. Moreover, the fuelconsumption is reduced, which can be attributed, inter alia, to reducedgas exchange work on account of lower exhaust gas mass flows.

The intake and/or exhaust valve may be one of the gas exchange valves oran additional valve. The former option allows a more compact form of theinternal combustion engine, whereas the latter, on account of mechanicaldecoupling, ensures adjustment within wider ranges.

The intake and/or exhaust valve for exhaust-gas recirculation can becontrolled by way of the camshaft. This merely requires an additionalcam on the camshaft. This may be arranged in front of the intake cam orbehind the exhaust cam. The successive flanks of the additional cam forthe exhaust-gas recirculation and of the cam for the gas exchange shouldin this case not intersect one another, i.e. the descending flank of theadditional cam and the rising flank of the intake cam or the descendingflank of the exhaust cam and the rising flank of the additional camshould not intersect one another. If appropriate, an adjustable camshaftcan also be used to adapt the control times to the engine state.

Alternatively, the intake and/or exhaust valve for exhaust-gasrecirculation can also be electromechanically controlled.

If the intake or exhaust valve is an additional valve, an electricalcontrol device is electrically connected to an electromagnetic,electromechanical, electrohydraulic, electropneumatic, etc. actuatorwhich is known per se and acts on the additional valve directly or, toachieve a flatter overall form and increased actuating force, via adiverter lever or the like. The control decoupled from the camshaftallows control as a function of operating state, so that the valve liftcurves can be optimally set at any time. For this purpose, acharacteristic diagram which gives the control times and the stroke forthe valves as a function of the operating parameters (e.g. load state)of the internal combustion engine can be stored in the electricalcontrol device.

If the intake or exhaust valve is a gas exchange valve, anelectromagnetic actuator is provided for adjusting it, this actuatoracting on one limb of a diverter lever which is mounted in the cylinderhead and the other limb of which is coupled via a free-wheel to the freeend of one of the gas exchange valves, allowing camshaft-controlledopening of the gas exchange valve without adjustment of the diverterlever.

The adjustment via a free-wheel decouples the actuator actuation of thegas exchange valve from the camshaft actuation. This allows optimumcontrol, in particular as a function of operating state, of theadditional actuation of the gas exchange valve. The exhaust-gas contentin the mixture for the internal exhaust-gas recirculation can thereforebe set optimally at any time even without an additional valve, inparticular taking account of the load state of the internal combustionengine.

Furthermore, further control options are opened up. By way of example,the electromechanical control of the exhaust valve allows an enginebraking function to be implemented. In this case, the exhaust valve isbriefly opened during compression, in order to allow controlleddecompression of the cylinder and to brake the internal combustionengine. The desired braking action can be set by the opening and closingtime and the stroke of the exhaust valve independently of the actuationof the gas exchange valves.

The diverter lever can be used to transmit high forces to the gasexchange valve. This is particularly advantageous for the engine brakingfunction, in which an exhaust valve has to be opened during compressioncounter to the high cylinder internal pressure. Moreover, the diverterlever allows a flat overall form.

The valve-side limb of the diverter lever can actuate the free end ofthe gas exchange valve directly or indirectly, for example via a valvebridge or a rocker lever. Direct actuation of a gas exchange valve is ofsimple design and allows the highest possible actuating force, making itparticularly suitable for the engine braking function. If, by way ofexample, the limb engages on a valve bridge via a rocker lever, it ispossible for a plurality of gas exchange valves to be actuatedsimultaneously, albeit with the force divided.

The free-wheel is preferably a bolt which can be displaced in a sleeveas an extension of the axis of the gas exchange valve and which may bearranged on the valve-side limb of the diverter lever. As a result, thediverter lever is in guided engagement with the gas exchange valve or ifappropriate the valve bridge or the like. The free-wheel may also be abearing means on the gas exchange valve side, against which a projectionon the valve-side limb of the diverter lever comes to bear.

BRIEF DESCRIPTION OF THE DRAWINGS

Further configurations of the invention are given in the followingdescription and the subclaims.

The invention is explained in more detail below on the basis ofexemplary embodiments illustrated in the appended figures. In thedrawing:

FIG. 1 shows a sectional view through a cylinder of a four-strokeinternal combustion engine.

FIGS. 2 and 3 show valve lift curves for the intake and exhaust, thevalve stroke being given as a function of the crank angle and theignition TDC lying at 0° and the gas exchange TDC at 360°.

FIG. 4 illustrates a camshaft-independent valve control.

FIG. 5 shows a camshaft cross section.

FIG. 6 shows a camshaft portion.

FIG. 7 shows a diagram in which the transfer of exhaust gas inaccordance with the invention into the intake duct and from the latterinto the cylinder is plotted, by way of example, as mass flow againstthe crank angle for various engine speeds.

FIG. 8 shows power, exhaust-gas recirculation rate and relative airratio plotted against engine speed in diagrammatic form.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The cylinder 1, which is partially illustrated in FIG. 1, of a pluralityof cylinders of a four-stroke internal combustion engine can beconnected via an intake valve 2 to an intake duct 3 for fresh gas andvia an exhaust valve 4 to an exhaust duct 5 for exhaust gas.

The internal combustion engine successively passes through the strokesof intake, compression, combustion and exhausting, with the strokes ofexhausting and intake representing the gas exchange. For this purpose,the valves 2, 4 are actuated periodically in a known way, in particularunder camshaft control. During intake, the valve lift curve 6, which ispartially illustrated in FIG. 2, results for the intake valve 2, andduring exhausting the valve lift curve 7 results for the exhaust valve4; the two valve lift curves 6, 7 may overlap one another in a known wayin order for the exhaust gas to be completely discharged from thecylinder 1.

For internal exhaust-gas recirculation, the intake and/or exhaust valve2, 4 is/are, moreover, actuated in addition to and independently of thegas exchange.

Additional actuation of the intake valve 2 during exhausting serves toadmit part of the exhaust gas formed during combustion to the intakeduct 3, in order for a mixture of fresh gas and exhaust gas to beintroduced into the cylinder during the next intake stroke. To set theexhaust-gas quantity, it is possible to adjust the opening and closinginstants and the valve stroke of the intake valve 2 accordingly, cf. thevalve lift curves 8. It is expedient for the intake valve 2 to open witha short stroke just after the exhaust valve 4 and to close well beforethe latter, cf. the valve lift curve 9. However, it is also possible forthe intake valve 2 to open before the exhaust valve 4 or at the sametime as it.

In addition or as an alternative, the exhaust valve 4 can be openedbefore or during the intake, so that not just fresh gas but also exhaustgas passes into the cylinder 1. In this case too, valve lift curves 10,cf. FIG. 3, may have different opening and closing instants and valvelifts depending on the boundary conditions. It is expedient for theexhaust valve 2 to open with a short stroke just after the intake valve4 and to close well before the latter, cf. the valve lift curve 11.

As an alternative to the actuation of the intake and exhaust valves 2, 4used for gas exchange, it is possible to provide one or more additionalvalves for exhaust-gas recirculation. These additional valves are ineach case connected to the intake or exhaust duct 3, 5, expediently viaan additional connecting passage.

An electromagnetic actuator 12 which is known per se may be arranged inthe cylinder head for actuation of the intake and/or exhaust valve 2, 4whether as a gas exchange valve or as an additional valve, cf. FIG. 4.The actuator 12 engages, via its longitudinally displaceable bolt 13, ona limb 14 of a diverter lever 15 mounted in the cylinder head. The otherlimb 16 of the diverter lever 15 acts via a free-wheel on a valve 17that is to be controlled. The free-wheel is in this case formed by abolt 18 on the limb 16, which is guided in a valve bridge 20, actingdirectly on a valve stem end 19, in such a manner that it can bedisplaced in sliding fashion as an extension of the valve axis. As aresult, the valve 17 can be actuated both for the knowncamshaft-controlled gas exchange via a rocker lever 21 and the valvebridge 20 and, independently of this, via the diverter lever 15 forexhaust-gas recirculation or engine braking or the like. The diverterlever 15 can also act on the valve-side end of the rocker lever 21, sothat the valves 17, 17′ illustrated here can be actuated together.

In this case, an electrical control device can be provided forcontrolling the opening and/or closing time and/or the valve stroke ofthe intake and/or exhaust valve 2, 4. As a result, the engine brakeand/or the internal exhaust-gas recirculation can be switched onaccording to demand and controlled as a function of operating state. Thevalve lift curves 8, 10 can therefore always be optimally matched to thecurrent load state etc. of the internal combustion engine. For thispurpose, there is expediently an electronic memory in which is stored acharacteristic diagram from which the opening and/or closing time and/orthe valve stroke can be called up as a function of the operating stateof the internal combustion engine in order to achieve the desiredexhaust-gas recirculation rate or the desired engine braking effect.

The actuator 12 may have a permanent magnet which acts in the directionof deflecting the bolt 13, assisting an electrically actuated closingmagnet in the actuator 12, so as to reduce the power required fordeflection.

As an alternative to using the actuator 12, it is also possible for theactuation of the intake and/or exhaust valve 2, 4 for exhaust-gasrecirculation to be camshaft-controlled. For this purpose, a camshaft 22has both a cam 23 for the gas exchange and an additional cam 24 for theexhaust-gas recirculation, cf. FIG. 5, which cams may, as illustrated,be spaced apart from one another along the longitudinal axis of thecamshaft 22, cf. FIG. 6. In the case of internal combustion engines withtwo camshafts, the cam 23 may be arranged on one camshaft and theadditional cam 24 on the other camshaft.

If the cams 23, 24, contrary to what is illustrated in FIG. 5, arearranged at one location on a camshaft, a separate valve adjustment isrequired for each cam 23, 24, allowing independent actuation of theassociated valve. This can take place analogously to the adjustmentillustrated in FIG. 4. In particular, the cams 23, 24 may each beassigned a rocker lever, in which case both rocker levers actindependently on a valve stem end or a valve bridge.

The diagram illustrated in FIG. 7 plots, by way of example, curves fordifferent engine speeds at full load in the situation in whichadditional opening of the respective intake valve 2 (IVO) is beingstarted by the additional cam 24 at 150° crank angle (° CA) after topdead center of the ignition, while the opening of the exhaust valve 4(EVO) takes place at 104° CA, the curves plotting the transfer ofexhaust gas into the intake duct or out of it into the cylinder 1 asmass flow [g/s] against crank angle [° CA]. The curve parts below thezero axis represent the transfer of exhaust gas out of the respectivecylinder 1 into the intake duct 3, while the curve parts above the zeroaxis represent the transfer of exhaust gas back out of the intake duct 3into the cylinder 1 during the opening period of the intake valve 3caused by the additional cam 24.

With regard to the internal exhaust-gas recirculation by additionalopening of the intake valve 3 by means of the additional cam 24, FIG. 8shows a diagram plotting a calculated comparison between power in %,exhaust-gas recirculation rate (EGR rate in %) and relative air ratiolambda_(v), plotted against the engine speed in %, between opening ofthe intake valve 3 at approximately 180° CA after top dead center of theignition (cf. WO 03/040540), dashed curves, and opening at 120° CA aftertop dead center of the ignition (in accordance with the invention),solid curves, under otherwise identical conditions (EVO at 85° CA). Thelatter is shown in the diagram in which the power is plotted against theengine speed, since in that diagram both curves coincide. The two otherdiagrams show that opening at approximately 180° CA leads tosignificantly lower exhaust-gas recirculation quantities, specificallyat approximately 70% of rated engine speed there is virtually noexhaust-gas recirculation any more, even though this is required at thispoint and is also delivered according to the invention, so thatsmoke-free combustion ensues over the entire engine speed range.Although opening at approximately 180° CA leads to high relative airratios, these are not required.

1. A method for internal exhaust-gas recirculation in an internalcombustion engine with gas exchange valves periodically controlled by acamshaft in accordance with the four-stroke principle, in which method,during the gas exchange exhausting of exhaust gas from the respectivecylinder into the exhaust duct by opening the exhaust valve, an intakevalve associated with the respective cylinder is additionally opened ina crank angle range after top dead center of the ignition of the gasmixture present in the cylinder, in order to exhaust part of the exhaustgas into an intake duct, so that during the next gas exchange intakeboth fresh gas and exhaust gas are taken in from the intake duct, therespective intake valve being actuated during the gas exchangeexhausting in addition to the periodic gas exchange intake,characterized in that the start of additional opening of the respectiveintake valve is carried out in the range from 110 to 150% of the openingangle of the start of opening of the exhaust duct by the exhaust valve.2. The method as claimed in claim 1, characterized in that the start ofadditional opening of the respective intake valve is carried out in therange from 110 to 130% of the opening angle of the start of opening ofthe exhaust duct by the exhaust valve.
 3. The method as claimed in claim1, characterized in that the intake valve and/or the exhaust valveis/are electromechanically actuated.
 4. The method as claimed in claim3, characterized in that the opening and/or closing time and/or thevalve stroke of the intake and/or exhaust valve is/are controlledaccording to the operating state of the internal combustion engine. 5.The method as claimed in claim 3, characterized in that the openingand/or closing time and/or the valve stroke is/are called up from anelectronic memory.
 6. An internal combustion engine with gas exchangevalves periodically actuated by a camshaft in accordance with thefour-stroke principle for connecting a cylinder to an intake duct duringgas exchange intake and to an exhaust duct during gas exchangeexhausting, in which there are an exhaust valve and an intake valve,which is additionally opened during the gas exchange exhausting ofexhaust gas from the cylinder into the exhaust duct, in order for partof the exhaust gas to be exhausted into the intake duct, so that duringthe next gas exchange intake both fresh gas and exhaust gas are taken infrom the intake duct into the cylinder, the start of additional openingof the respective intake valve being provided in the range from 110 to150% of the opening angle of the start of opening of the exhaust duct bythe exhaust valve.
 7. The internal combustion engine as claimed in claim6, characterized in that there is a control device for actuating theexhaust valve during the gas exchange intake independently of theperiodic gas exchange exhausting and/or for actuating the intake valveduring the gas exchange exhausting independently of the periodic gasexchange intake.
 8. The internal combustion engine as claimed in claim6, characterized in that an electromagnetic actuator is connected to anelectronic control device and acts on a limb of a diverter lever, whichis mounted in the cylinder head and the other limb of which is coupled,via a free-wheel, to the free end of one of the gas exchange valves,allowing camshaft-controlled opening of the gas exchange valve withoutadjustment of the diverter lever.
 9. The internal combustion engine asclaimed in claim 7, characterized in that the control device is providedfor opening and closing the intake and/or exhaust valve during the gasexchange exhausting or gas exchange intake from a closed position. 10.The internal combustion engine as claimed in one of claims 6 or 7,characterized in that the intake valve and/or the exhaust valve is/aregas exchange valves.
 11. The internal combustion engine as claimed inone of claims 6 or 7, characterized in that the intake valve and/or theexhaust valve are additional valves which can be closed during the gasexchange intake and/or the gas exchange exhausting involved in the gasexchange.
 12. The internal combustion engine as claimed in claim 7,characterized in that the control device comprises an additional cam onthe camshaft.
 13. The internal combustion engine as claimed in claim 7,characterized in that the intake and/or exhaust valve is anelectromechanically actuable valve and the control device is a circuit.14. The internal combustion engine as claimed in claim 13, characterizedin that the control device is designed to control the opening and/orclosing time and/or the valve stroke of the intake and/or exhaust valveaccording to the operating state of the internal combustion engine. 15.The internal combustion engine as claimed in claim 14, characterized inthat an electronic memory is provided, in which the opening and/orclosing time and/or the valve stroke is/are stored as a function of theoperating state of the internal combustion engine.
 16. The internalcombustion engine as claimed in claim 8, characterized in that thefree-wheel is a bolt which can be displaced as an extension of the axisof the gas exchange valve.
 17. The internal combustion engine as claimedin claim 15, characterized in that the bolt is arranged on thevalve-side limb of the diverter lever.
 18. The internal combustionengine as claimed in claim 8, characterized in that the valve-side limbof the diverter lever is in engagement with the free end of the gasexchange valve.
 19. The internal combustion engine as claimed in claim8, characterized in that the valve-side limb of the diverter lever is inengagement with a valve bridge or a rocker lever for the gas exchangevalve.
 20. The use of the internal combustion engine as claimed in claim6 for engine braking.